The instant invention relates generally to marine screw propellers and more particularly to a propeller using the circulation control principle of blowing tangentially over a coanda surface to develop higher than usual thrust as well as to control the direction of that thrust.
Marine screw propellers used in the propulsion of vessels such as ships, submarines, and the like generally have a fixed pitch and are designed primarily for forward propulsion. In order to stop the ship or to back it, it is necessary to reverse the rotation of the propeller. The procedure and machinery for stopping and backing large ships is extensive. Either the reduction gear requires a reverse gear, or a reversing engine or turbine must be provided. Stopping and backing of a ship is very time consuming and therefore must be anticipated by the conning officer because of the time involved. The complete propulsion machinery system including the propeller, propeller shaft, reduction gear, and turbine or engine must be brought to a complete stop while the ship coasts. Then the propeller rotation and therefore its thrust is reversed by reversing all the propulsion machinery.
In applications where propeller thrust reversal is required repeatedly and rapidly, such as in ice breaking operations, reversible pitch propellers have been devised. These propellers are not particularly efficient because of their shape and the necessary protuberances of the variable pitch mechanism.
Regarding propeller efficiency and noise reduction, prior work has focused on blade form and means to reduce or eliminate cavitation. Previous attempts to reduce cavitation involve discharging air or water from the back or the trailing edge of the blades to merely fill the vacuum void, and not to increase the thrust produced by the blade.